Re: [PATCH RFC v2 6/9] xen/arm: Add handling write fault for dirty-page tracing

[Ian Campbell <ian.campbell@citrix.com>]

On Fri, 2013-07-05 at 13:42 +0900, Jaeyong Yoo wrote:
> > > > I think we'd want to establish a linear map of the current guest p2m
> > > > for these purposes so we could also avoid all the map_domain_page
> > > > stuff in the fault path. I think there is enough virtual address
> > > > space left for that even on 32-bit, it's less critical on 64-bit
> > > > anyway since we are about to have a direct map of RAM available to us.
> > >
> > > I think it is a good idea to have an ever-mapping linear dirty-bit map
> > > for guest p2m.
> > 
> > By Linear map I meant slotting the p2m into the hypervisor's own page
> > tables (I believe the relevant p2m and pte bits in the entries do not
> > overlap, so this is possible), such that the p2m table entries are
> > available at known virtual addresses and you can locate the L1, L2 and
> > L3 entries corresponding to a given guest physical address with just
> > arithmetic, i.e. something like:
> > http://www.technovelty.org/linux/virtual-linear-page-table.html
> 
> Which virtual address range should we map the p2m? Would it be xenheap, 
> domheap, or unassigned range (for instance, between  frametable and vmap)?

Since it needs to take over a bunch of entries in the page tables it
should be its own virtual address range. Looking at
xen/include/asm-arm/config.h between the frametable and vmap seems
sensible.

Ian.